Mass spectrometry has become an essential art in such fields as microanalysis of environmental pollutants, structure determination of proteins and other bimolecular or molecular aggregates, etc. There are three problems in mass spectrometry: 1) introduction (interfacing) into vacuum and vaporization/ionization of a sample; 2) attainment of high mass resolution; and 3) attainment of high sensitivity; and various methods have been proposed for the respective problems.
For applying mass spectrometry to various samples, development of new arts concerning 1) above is essential. Though gaseous samples, volatile samples, and samples that are readily vaporized by heating can be introduced into vacuum after vaporization and ionized using a laser, electron gun, etc., vaporization and ionization without fragmentation of the sample is a major issue for non-volatile samples and pyrolytic samples. Presently, the following two types of methods are mainly put to practical use as methods for vaporizing non-volatile samples and pyrolytic samples.
Firstly, there are methods of performing instantaneous heating and vaporization using a laser (laser desorption method). These methods were originally developed to vaporize metals. When a laser is focused onto a sample, the sample is heated to several thousand degrees Celsius instantaneously and is thereby vaporized. However, when a laser method is applied likewise to an organic molecule, dissociation of the molecule occurs due to multiphase absorption and large amounts of fragments are generated, making analysis of a mass spectrum difficult.
As a method of resolving this problem, a sample can be imbedded in a matrix and laser light whose wavelength is fixed to an absorption band of the matrix can be used to prevent the dissociation of the sample. This method is called the matrix-assisted laser desorption ionization (MALDI) method.
In the other type of method, a solution of a sample is prepared and the sample is vaporized by removing the solvent. In the case of electrolytic samples, a method is used in which an electro spray is used to take ions out from the solution and introduce the ions into vacuum via an orifice (electro spray ionization (ESI) method).
There is also the thermo spray (TS) method, in which a sample solution is introduced and vaporized in a capillary, heated by a heater, and then sprayed into vacuum.
With these methods, a key to technical development lies in how the solvent is removed, and as methods of using a solvent that is more readily removed, there are methods of using supercritical fluids (liquefied gases) (supercritical fluid mass spectrometry (SCF-Mass)) In such a method, when a supercritical fluid solution of a sample is introduced into vacuum via an ultrafine capillary, the solvent and the sample vaporize immediately.
A method of performing mass spectrometry of molecules using a supercritical fluid (liquefied gas) of carbon dioxide, etc., is disclosed in Non-Patent Document 1 mentioned below, etc.
In the abovementioned MALDI method and ESI method, which are know as methods of ionizing a vaporized sample, vaporization and ionization of the sample are carried out simultaneously. With a general mass analyzer, which is used upon connection to a gas (liquid) chromatograph, or with the TS method or the SCF-Mass method, etc., a gaseous sample must be ionized by some method.
A most generally used method is the electron impact ionization method, in which a discharge or an electron gun is used. Though this method is inexpensive and enables easy maintenance in terms of device, because the excess energy that is applied to molecules in the ionization process is extremely large, it is difficult to avoid dissociation (decomposition) of sample molecule. A large number of fragment peaks thus appear in a mass spectrum and extremely troublesome analysis is required.
On the other hand, with the laser ionization method, excess energy in the ionization process can be restrained readily and thus the dissociation of sample molecules can be lowered by adjustment of the laser wavelength. This method is thus referred at times as the soft/intact ionization method.
However, the mass spectrometry method gives information of only molecular masses, thus isomers of sample molecules or molecular aggregates obviously cannot be distinguished, and detailed information on molecular structures of sample molecules or molecular aggregates cannot be obtained from just the molecular mass data. However, by using the laser ionization method, in which vaporized sample molecules or molecular aggregates are ionized by a laser, various laser spectroscopic techniques can be applied to the vaporized sample to enable extremely detailed information to be obtained on the molecular structures of sample molecules or molecular aggregates as well as enable separation and observation of isomers of sample molecules or molecular aggregates using differences in electronic transition energy.
However, if the sample molecules or molecular aggregates are thermally distributed among various vibration states, an extremely complex electronic spectrum is obtained because various electronic transitions from different initial states are observed simultaneously, and not only analysis of the spectrum becomes difficult but the molecular selectivity is lowered as well. To resolve this problem, the sample molecules may be cooled and put in the lowest energy level. This is enabled by a supersonic jet technique. When a mixed gas of a sample gas and a carrier gas, such as a noble gas, is adiabatically expanded in vacuum via an orifice, a supersonic jet containing the vaporized sample molecules is generated.
It is known conventionally, from research by the present inventors described in Non-Patent Document 2 mentioned below, that by jetting a mixed gas, obtained by mixing helium gas or other carrier gas with a “volatile substance (the volatility is determined by the vapor pressure unique to a sample)” that can form a mixed gas, into vacuum from an orifice, vaporized sample molecules in an ultra cold state (the lowest energy level) without intermolecular collisions can be obtained to enable recognition of internal energy levels of the sample molecules.
Also in Japanese Published Unexamined Patent Application No. 2003-329556 (Patent Document 1) is disclosed a molecular beam generating method and device, with which a neutral molecular beam of a wide range of types of molecules, in particular, molecules that decompose upon high temperature heating or molecules that do not volatilize even when heated to a high temperature can be generated, and the molecules and molecular aggregates contained in the generated neutral molecular beam can be ionized to enable mass spectrometry, spectroscopic measurement, etc.
Non-Patent Document 1. T. Sakamoto, A. Yamamoto, M. Owari, and Y. Nihei, “Development of a Supercritical Fluid Extractor Coupled with a Time-of-Flight Mass Spectrometer for Online Detection of Extracts,” Analytical Sci. 19, 853 (2003).
Non-Patent Document 2. S. Ishiuchi, K. Daigoku, K. Hashimoto, and M. Fujii, “Four-color hole burning spectra of phenol/ammonia 1.3 and 1:4 clusters,” J. Chem. Phys. 120, 3215 (2003).
Patent Document 1. Japanese Published Unexamined Patent Application No. 2003-329556